Rectangular response optical filter for partitioning a limited spectral interval

ABSTRACT

The invention concerns a rectangular response optical filter for partitioning a limited spectral interval in a light flux with large spectrum comprising: a preferably monomode input optical fiber having one end; an array-reflector assembly in Litmann-Metcalf configuration; a converging optical system collimating at whose focus is set the input fiber end; a converging focusing optical system set between the array and the reflector; one or several output fibers of the same type as the input fiber. At least one reflector is placed in the focal plane of the focusing optical system and has a limited dimension in the dispersion plane, the position and the limited dimension of the dispersion plane determining the partitioned spectral interval.

This invention concerns a rectangular response optical filter forpartitioning a limited spectral interval and having optical fibres,preferably single mode fibres, as input and output gates.

The evolution of wavelength multiplexed optical fibre telecommunicationsrequires the development and the optimisation of such devices. It hasbeen sought in particular to partition a wide spectrum into spectraldomains, here called limited spectral intervals, while avoiding anysuperimpositions and cross-talk that might derive therefrom.

Numerous devices have already been proposed to that effect, whereas mostof them consist in spreading the luminous spectrum in a plane and inplacing in the said plane, a slot delineating the narrow spectral bandthat one wishes to select, but then the light cannot be recoupledefficiently in a monomode optical fibre.

Other devices implement sets of optically guided components: couplers,multiplexers-demultiplexers, . . .

The optimisation of such devices implies the provision of rectangulartransmission functions and without loss in the limited spectral intervallight flux selected, i.e. in a representation of the intensity of thelight flux transmitted as a function of the wavelength as that on FIG.1, the edges of the partitioned band should be as vertical as possible,the apex should be as flat as possible and the losses as little aspossible. The apex can be flattened according to the state-of-the-art bygenerating losses.

We also know a document (I. Nishi and al., December 1987) that divulgesa wide-band multiplexer-demultiplexer for multimode filter. It suggeststhe implementation of a retrodispersing system in Littrow configurationwith respect to an input fibre and to output fibres. This documentspecifies that the width of the pass-band of such a device is determinedby the length of the retroreflector.

Besides, in a published article (Chi-Luen Wang and al., 1994), isdescribed an external cavity laser wherein the external cavity is set upso that it enables filtering of two wavelengths. Filtering is performedby reflecting bands forming reflecting mirrors cooperating with agrating.

The implementation of the teachings of these documents does not enableto realise a transmission-stable device and ensuring good accuracy.

The inventors have set themselves the target of providing such a deviceimplementing a grating-reflector assembly in Littman-Metcalfconfiguration in order to take advantage of the high performancesoffered by such a type of configuration and that such a device does notgenerate any losses and possesses optical fibres, preferably monomode asinput and output gates, ensuring optimized stability and accuracy.

Thus, the invention concerns a rectangular response optical filter forpatitioning a limited spectral interval in a wide spectrum light fluxcomprising:

an input optical fibre having one end,

a grating-reflector assembly in Littman-Metcalf configuration,

a converging collimation optical system at whose focal point is locatedthe end of the input fibre,

a converging focusing optical system placed between the grating and thereflector,

at least one reflector placed in the focal plane of the focusing opticalsystem whose dimension is limited in the dispersion plane, whereas theposition and limited dimension of the reflector in the dispersion planedetermine the partitioned spectral interval.

According to the invention, the optical filter comprises a polarisationseparator placed between the input fibre and the grating and generatingtwo elementary light beams parallel and polarised orthogonally withrespect to one another, whereas a plate λ/2 is placed on one of theelementary beams in order to generate two elementary parallel beamspolarised in a direction perpendicular to the lines of the grating,whereas the reflector of Littman-Metcalf configuration is sending eachelementary beam back to path and in opposite direction in relation toone another.

In different embodiments each exhibiting its own specific advantages andliable to be used in the compatible technically combinations:

the input optical fibre is a monomode fibre,

the light flux generated with limited spectrum is collected in an outputoptical fibre distinct from the input fibre and of the same type as thelatter,

the optical filter comprises several optical output fibres, eachconnected to a reflector, whereas these reflectors are positioned in thefocal plane of the focusing optical system and have a small dimension inthe dispersion plane while determining a particular spectral interval,

the light flux generated with limited spectrum is collected by the inputfibre and the latter carries an optical circulator enabling to separatethe output flux from the incoming flux without any energy loss,

the optical filter comprises a folding reflector doubling the number ofpassages of the light beam on the grating,

the reflector of Littman-Metcalf configuration is a planar mirrorconnected to a bi-prism,

the reflector of Littman-Metcalf configuration is a truncated dihedron.

The invention will be described in more detail with reference to theappended drawings wherein:

FIG. 1 represents a spectrum partitioned by the device of the invention;

FIG. 2 represents a device of the invention implemented with acirculator;

FIGS. 3A and 3B represent a Littman-Metcalf configuration usedconventionally;

FIGS. 4A and 4B represent a first embodiment of the invention;

FIGS. 5A and 5B represent a first embodiment of the invention, withcompensation of the polarisation effects due to the grating;

FIGS. 6A and 6B represent a second embodiment of the inventionimplementing an output fibre distinct from the input fibre;

FIG. 7 is a detailed view of a reflector implemented in the secondembodiment;

FIG. 8 is a detailed view of an alternative reflector type that can beimplemented in the second embodiment;

FIGS. 9A, 9B and 9C represent a fourth embodiment of the invention;

FIG. 10 is an embodiment of the invention implementing an alternativereflector in the first embodiment of the invention.

FIG. 1 is therefore a diagram representing the energy of the light fluxcoming from the partitioning device of the invention, as a function ofwavelength λ. The incoming spectrum extends supposedly over a longrange, in wavelength, on the basis of the scale of this extended diagramand the device of the invention enables to partition a narrow bandrepresented by a function as close as possible to a rectangularfunction, with width Δλ centred on a wavelength λ_(i).

The device of the invention comprises therefore an input fibre 1 havingone end 2. The partitioning device of the invention as a whole isdesignated by the reference 3. This device comprises a grating4-reflector 5 assembly in Littman-Metcalf configuration.

We know that in the conventional Littman-Metcaif configuration,represented on FIGS. 3A and 3B, the incident collimated beam describesan angle θ₁ with respect to the normal to the grating. A reflector R isplaced with its normal having an angle θ₂ to the grating. The wavelengthλ that complies with the condition λ=p(sin θ₁+sin θ₂) where p is thepitch of the grating, is dispersed by the grating at an angle θ₂ thenretroreflected by the reflector that is then perpendicular to thelatter. Finally, it is dispersed again in the grating on the way backand comes out under the input angle θ₁. The wavelength λ is thereforeselected by the cavity. That wavelength λ can be varied while changingthe orientation of the grating-reflector assembly, i.e. while changingθ₁ or while changing solely the orientation of the reflector, i.e. whilechanging θ₂ or finally while changing solely the orientation of thegrating, i.e. while changing θ₁ and θ₂ while keeping θ₁-θ₂ constant.

In the description of such devices, it is customary to call dispersionplane the plane perpendicular to the lines of the grating containing thecentral ray of the incident beam and the central rays of the beamsdispersed by the grating, it is shown on FIG. 3B.

For each beam, transversal plane shall designate the plane perpendicularto the central ray and longitudinal plane shall designate the planeperpendicular to the dispersion plane containing the central ray. Thelongitudinal plane is therefore that on FIG. 3A.

On the various appended figures, view A is an unfolded view, i.e.wherein the beam dispersed 7 by the grating 4 has been represented inthe direct extension of the incident beam 6 for better readability. ViewB is a representation from beneath, i.e. in a plane parallel to thedispersion plane.

FIG. 4 with its views A and B shows a system wherein the selection inwavelength is conducted by the geometrical dimensions of a mirrorconnected to a grating in Littman-Metcalf configuration. A convergingoptical system 8 at whose focal point is placed the end 2 of the inputfibre 1, collimates the beam 9 emitted from the end of the fibre, sothat the incident beam 6 on the grating is a collimated b am. Thus, thdispersed beam(s) 7 are also collimated b ams and a converging opticalsystem 10 focuses these beams in its focal plane 4′ wherein is placed amirror 5 which has a limited dimension d in the dispersion 7′ plane asshown on FIG. 4B. For the wavelengths corresponding to the beamsreflected on the mirror, the system behaves like a cat's eye, and hencethese wavelengths are re-coupled on the way back in the input fibre,regardless whether it is monomode or multimode.

Thus, this mirror only reflects towards the optical system 10 and hencetowards the grating 4, a limited portion of the spectrum, whereas thewavelengths corresponding to the external beams 7″ are not reflected.

This flux is partitioned and coupled on the way back by the opticalsystem 8 then by the optical fibre 1 which therefore in that embodiment,acts as an input and output optical fibre.

Different devices can be considered to separate the input fluxes and theis output fluxes so that, in particular, preferably, FIG. 2 shows acirculator that enables to realise such separation with minimal energylosses.

The input-output fibre 1 connected to the partitioning device 3 istherefore connected to its other end to the circulator 11 whichpossesses an input 12 and an output 13.

This wavelength selection device operates correctly, but is stillproving unstable and is providing inaccurate light fluxes or signals.

To remedy these shortcomings, we have endeavoured, according to theinvention, to break free from the polarisation defects.

Thus, as represented on FIGS. 5A and 5B, we have implemented a devicecompensating for the known polarisation effects liable to be induced bythe grating 4 and to generate spurious effects.

To that effect, the collimated beam 6 emerging from the optical system 8is divided by the polarisation separator 14 into two parallel beams,respectively 15 and 16, with cross polarisation. A plate λ/2 17 modifiesthe flux polarisation 16, so that the flux 15 and the modified flux 18are polarised in a similar fashion and undergo therefore exactly thesame effects from the grating 4. The lens 10 causes each of these beamsto converge onto the mirror 5 which exchanges their paths, which meansthat the return paths of the beams 18 and 15 are exchanged afterreflection onto the mirror 5, whereas the beam 18 follows the opticalpath of th beam 15 on its way out and vice-versa.

Thus, the b ams 18 and 15 are recombined on the way back and haveundergone exactly the same effects of the grating 4.

Thus, any spurious effect liable to be generated by the grating inrelation to the polarisation and the shape of the spectral distributionof the partitioned light flux is therefore improved

Different preferred embodiments enable the implementation of the devicedescribed above and each of them enhances the thinness of thepartitioned spectral band and possibly, in order to partition a greaternumber of elementary bands in the incident wide spectrum.

FIGS. 6A and 6B represent an embodiment wherein the output fibre 20 isdistinct from the input fibre 1.

To that effect, the mirror 5 is replaced with a reflector 21 which, seenin the longitudinal plane, has the shape of a dihedron whereas it keepsa small dimension d in the dispersion plane.

As represented in the longitudinal plane, this dihedron 21 is positionedwith respect to the converging optical system 10 so that afterreflection onto each of the faces of the dihedron 21, the parallelincoming beams in the optical system 10 converge into a beam 41 in themiddle plane 22 of the dihedron 21 and emerge in the shape of asymmetrical beam 42, enabling as well as the beam 23 transmitted by thefibre 1, forming a beam 24 which is symmetrical to the beam 23 withrespect to the optical axis 25 of the system and is received by thefibre 20 set symmetrically with respect to that axis of the input fibre1.

Such a dihedron is represented more in detail on FIG. 7 and thiscomponent can be replaced in a similar fashion by the assemblyrepresented on FIG. 8 consisting of a biprism 30 and a mirror 31. As themirror 31 is perpendicular to the axis of symmetry of the biprism 30, anincoming beam 41 generating the beam 33 by the deviation of the biprism30 is converging in the plane of the mirror 31 and reflectedsymmetrically. The mirror 31 generating a beam 32 which, after deviationby the biprism 30, produces a beam 42. The beam 42 is symmetrical to thebeam 41 This component 30, 31 therefore enables, as the reflector 21,the realisation of a beam 35 to be received by the fibre 20 from thebeam 23 transmitted by the fibre 1.

FIGS. 9 (9A, 9B, 9C) represent an embodiment of the invention enablingsimultaneously to compensate for the biasing effects as stated above, tolin arise the distribution of the spectrum, in frequency, in thepartitioning zone and to compensate for the anamorphosis normallyinduced by the grating.

To that ffect, a polarisation separator is placed after the conv rgingoptical system 8 and breaks down the incident light beam 9 generated bythe input optical fibre 1 into two beams 15 and 18. A prism 27 is thenplaced on the beams and realises a first dispersion before that producedby the grating 4.

We know that it is thus possible to generate, thanks to the associationof the prism 27 and of the grating 4, a frequency-linear dispersion.

The light beams are then folded back onto themselves by a reflector 26which therefore sends them back, in reverse direction, onto thedispersing assembly formed by the grating 4 and the prism 27.

For better readability, FIG. 9 shows independently, on view A, atransversal representation of the device as the views A of FIGS. 3, 4,5, 6, on view B, a view in the dispersion plane corresponding to theupper stage of view A and, on view C, a view of this same dispersionplane of the lower stage of view A.

At the upper stage, after new dispersion by the grating 4-prism 27assembly, the optical collimating system 10 focuses these beams onto themirror 5 which proceeds to the requested spectral selection.

The beams selected are then reflected and follow a path reverse fromthat described until now to converge on the way back onto the end 2 ofthe fibre 1.

Thus, the polarisation separation enables symmetrical action of thegrating during each of these passages and avoids therefore any spuriouseffect, the association of a prism and of a grating enables frequencylinearization in the spreading plane of the spectrum, i.e. in the planeof the mirror 5, the double passage of each of the beams through thedispersing assembly (grating-prism) ensures compensation for theanamorphosis and hence efficient coupling of the beam outgoing in thefibre 1. This fourth embodiment can be used in combination with thethird embodiment while replacing the single fibre by an input fibre andone or several output fibres and while replacing the mirror by one orseveral reflecting dihedral or mirror-biprism assemblies.

It may also be useful to associate each fibre with a microlens in orderto reduce the divergence of the beam 9.

Finally, this filter may be tuneable while modifying the position or thewidth d of the reflector, or while placing in rotation the grating orthe collimation optical system—reflector assembly—or finally the foldingreflector 26. Connected to a detector, this filter enables to realise ananalyser for rectangular spectral response optical spectrum.

FIG. 10 represents an embodiment of the invention wherein the outputfibre is distinct from the input fibre and wherein a reflector as thatrepresented and described by reference to FIG. 8, is used.

The elements represented on the previous Figures have been designated bythe same numerical references, as on FIGS. 6A and 6B, the beam 23transmitted by the fibre 1 forms a beam 24 symmetrical to the beam 23with respect to the optical axis 25 of the system.

The polarisation separator 14 splits the incoming beam 23 into twoparallel beams, respectively 15 and 16. After reflection onto the mirror31 and having been deviated by the biprism 30 before reflection as wellas after reflection, both these beams pass again through the assemblyformed by the plate λ/2 17 and the polarisation separator 14 in order toform the return beam 24 that is coupled to the optical fibre 20. Solelythe reflected beam 15′ from the beam 15 is subject to the plate λ/2 17.Conversely, the reflected beam 18′ from the incident beam 18 is sent tothe polarisation separator 14 without being subject to the effect ofthat plate λ/2. The beams 18′ and 16′ (generated from the beam 15′ bythe effect of the plate λ/2 17) are combined by the bias separator 14 toform the beam 24.

This device has been described with a mirror whose sizes and positionare fixed.

In certain applications, it may be useful to vary the spectral width ofthe selected flux and/or its central wavelength. In order to control thespectral width, a slit with variable width is placed before a largemirror. The position of the slit in its plane determines the centralwavelength.

What is claimed is:
 1. A rectangular response optical filter forpartitioning a limited spectral interval in a light flux with widespectrum comprising: an input optical fibre having one end, agrating-reflector assembly in Littman-Metcalf configuration, aconverging collimation optical system at whose focal point is locatedthe end of the input fibre, a converging focusing optical system placedbetween the grating and the reflector, at least one reflector placed inthe focal plane of the focusing optical system which has a dimensionlimited in the dispersion plane whereas the position and the limiteddimension of the reflector in the dispersion plane determine thepartitioned spectral interval, characterized in that it comprises apolarisation separator placed between the input fibre and the gratingand generating two elementary light beams parallel and polarisedorthogonally with respect to one another, whereas a plate λ/2 is placedon one of the elementary beams in order to generate two elementaryparallel beams polarised in a direction perpendicular to the lines ofthe grating, whereas the reflector of Littman-Metcalf configurationsends back each elementary beam onto the path and in opposite directionin relation to one another.
 2. An optical filter according to claim 1,characterised in that the input optical fibre is a monomode fibre.
 3. Anoptical filter according to one of the claims 1 and 2, characterised inthat the light flux generated with limited spectrum is collected in anoutput optical fibre distinct from the input fibre and of the same typeas the latter.
 4. An optical filter according to claim 3, characterizedin that it comprises several optical output fibres, each of them beingconnected to a reflector, these reflectors being positioned in the focalplane of the focusing optical system and having a small dimension in thedispersion plane and determining a particular spectral interval.
 5. Anoptical filter according to claim 4, characterized in that it comprisesa folding reflector doubling the number of passages of the light beam onthe grating.
 6. An optical filter according to claim 4, characterized inthat the reflector of Littman-Metcalf configuration is a planar mirrorconnected to a bi-prism.
 7. An optical filter according to claim 4,characterized in that the reflector of Littman-Metcalf configuration isa truncated dihedron.
 8. An optical filter according to claim 3,characterized in that it comprises a folding reflector doubling thenumber of passages of the light beam on the grating.
 9. An opticalfilter according to claim 3, characterized in that the reflector ofLittman-Metcalf configuration is a planar mirror connected to abi-prism.
 10. An optical filter according to claim 3, characterized inthat the reflector of Littman-Metcalf configuration is a truncateddihedron.
 11. An optical filter according to one of the claims 1 and 2,characterised in that the light flux generated with limited spectrum iscollected by the input fibre and in that said fibre carries an opticalcirculator enabling to separate the output flux from the incoming fluxwithout any energy loss.
 12. An optical filter according to claim 11,characterized in that it comprises a folding reflector doubling thenumber of passages of the light beam on the grating.
 13. An opticalfilter according to claim 11, characterized in that the reflector ofLittman-Metcalf configuration is a planar mirror connected to abi-prism.
 14. An optical filter according to claim 11, characterized inthat the reflector of Littman-Metcalf configuration is a truncateddihedron.
 15. An optical filter according to one of the claims 1 or 2,characterized in that it comprises a folding reflector doubling thenumber of passages of the light beam on the grating.
 16. An opticalfilter according to claim 15, characterized in that the reflector ofLittman-Metcalf configuration is a planar mirror connected to abi-prism.
 17. An optical filter according to claim 15, characterized inthat the reflector of Littman-Metcalf configuration is a truncateddihedron.
 18. An optical filter according to one of the claims 1 or 2,characterized in that the reflector of Littman-Metcalf configuration isa planar mirror connector to a bi-prism.
 19. An optical filter accordingto one of the claims 1 or 2, characterized in that the reflector ofLittman-Metcalf configuration is a truncated dihedron.